It is well-known to compress gases of all kinds, including elemental and other gases for scientific or industrial purposes, for transport and delivery to consumers or other customers. For example, it is known to compress natural gas and to transport the compressed natural gas (CNG) by truck, ship, or similar delivery system. As indicated in U.S. Pat. No. 6,339,996 to Campbell, there are users of natural gas that periodically require natural gas supply in excess of the supply available through existing pipelines. Further, there are areas in which natural gas service via pipeline is not available at all, due to remoteness, the high cost of laying pipelines, or other factors. In such cases, tanks of CNG transported by truck, for example, can be an economical way to provide the natural gas service required by such users.
To be economical, such tanks must be filled with large amounts of usable natural gas. Accordingly, full tanks of CNG are under very high pressure, commonly around 3000 pounds per square inch (psi). However, in many cases natural gas under considerably lower pressure, e.g. from 20 to 100 psi, is required. Consequently, unloading a CNG tank requires a substantial reduction in the gas pressure prior to being received at a customer's intake. Currently, that reduction takes a relatively long time, principally for two reasons. First, standard pneumatic regulators capable cannot reduce gas pressure at a high rate. Regulators that are capable of reducing pressure from 3000 psi to 100 psi must allow only a relatively small amount of gas through in a given time period in order to keep the downstream pressure stable. Second, according to the laws of chemistry a pressure decrease of a gas results in a proportional temperature drop, assuming constant volume. Allowing a large volume of CNG to be depressurized at once results not only in a great physical strain but also in a large temperature drop that can cause substantial damage to or malfunction of the CNG tank, valves, pipelines (particularly plastic or PVC pipes), customer equipment or other pieces of a natural gas system.
Users of large volumes of natural gas may require flow rates of 1000 cubic feet per hour (1000 cfh). At such rates, the cooling resulting from depressurization is considerable, as is the chance of significant or catastrophic failure if the pressure at the customer's intake is not stable and within the customer's specifications. Such failures could result in a loss of a substantial volume of gas through a relief valve that releases gas to atmosphere when pressure is too high. At worst, a failure could result in irreparable damage or destruction of equipment and/or explosion.
It is understood that there are electric or electronic devices, control valves, and/or pressure controllers that may be able to accept the high-pressure CNG, depressurize it, and pass it to a standard natural gas intake at a relatively high rate of delivery. Such devices are extremely expensive, however, reducing or eliminating the profitability of truck-delivery of CNG. Further, failures or other problems with such devices result in repairs or replacements that are quite expensive.
Accordingly, there is a need in the industry for a gas unloading system that is inexpensive, yet allows delivery of depressurized gas at a relatively high rate with proper safety.